Spin injection and detection in all-oxide nanostructures – SPINOXIDE

Spin-based technology is viewed as a promising field on the “beyond-CMOS” roadmap as it offers a possible solution to reduce the heat of IT devices as their dimensions shrink. Exploiting the spin instead of, or in addition to, the charge degree of freedom, could lead towards new multifunctional devices offering non-volatility, higher processing speeds, higher packing densities and reduced power consumption. While spintronics is a multi-disciplinary field, there is a worldwide effort to integrate semiconductors and magnetic materials, as, at the very fundamental level, an efficient spin injection and detection of spin in semiconductors is essential to the field, but remains an unsolved issue.
On the other hand, one possible way out to circumvent the electrical spin injection challenge is to use polarized light as this is done in All-Optical Spin Switching. The method relies on ultra-fast circularly polarized laser beams to reverse spins from one direction to the other deterministically and avoids electrical spin injection or the use of external magnetic fields. It is very efficient in some ferrimagnetic thin films such as GdFeCo but also in semiconductors if optical selection rules are obeyed. In the latter case, ultrashort pulses are not required since the semiconductor acts as a photodetector, converting the light helicity into a spin current.
The goal of SPINOXIDE is to explore the combination of a ferrimagnetic oxide having a high Curie temperature and spin polarization close to 100% with the semiconducting oxide ZnO not only to demonstrate efficient electrical spin injection/detection in semiconductors, but also to fabricate practical spin-optronics devices (spin-photodetectors and spin-light emitters) addressing the IT industry and with the potential of benefiting the display and the drug industries in a shorter term.
We have 3 main objectives in SPINOXIDE :
1) The first objective is to achieve practical spin polarization and injection into ZnO nanostructures as well as spin detection from ZnO nanostructures. To this end, we will investigate the combination of the ZnO semiconductor with high TC ferrimagnetic oxides (FMO), one being a semi-metal (Fe3O4) and the other a semiconductor (Fe1.5Ti0.5O3). In this system, the well-known “conductivity mismatch” limitation for the ferromagnetic metal/semiconductor interface should not apply. Consequently, there is a complete freedom for tuning the band alignments between ZnO and the FMO and spin injection could even be effective using ohmic contacts which have a much lower contact resistance than tunnel contacts

2) The second objective is to limit or even suppress the main spin relaxation mechanism at rt in ZnO. To this end, we will take advantage of the peculiarities of the spin-orbit Hamiltonian in wurtzite materials to spin-orbit engineered ZnO quantum well and nanowires and potentially increase the already very long spin coherence times in ZnO at 300K (among the longest for standard direct gap semiconductors) by several orders of magnitude.

3) The third objective is to capitalize on the knowledge gained to fabricate and test practical spintronic devices. To this end, the FMO with highest potential will be selected. Spin-photodetector, which can discriminate left from right-hand circularly polarized light, will be based on planar heterostructures, while spin-light emitters and spin-nanolasers, which will emit circularly polarized light, will be based on core-shell nanowires, having a magnetic shell. The use of ZnO NWs insures ultrafast lasing dynamics (>200GHz) and spots sizes 20 times smaller than that with materials emitting in the IR
The success of our approach relies on the assembly of individual spintronic building-blocks, mastered by the consortium partners independently, and on the management of well identified risks while preliminary results show a potential for the fabrication of nano spinlasers operating at rt without any external magnetic field.